The present invention relates to a colored composition for color filter which permits color display, and a display using color filters formed of the colored composition.
As thin and light color displays, flat panel displays such as liquid crystal displays, organic light emission displays and plasma display panels have been put to practical use or are under study for their practical application. For example, the liquid crystal displays have already been put to practical use as devices which fundamentally comprise two glass substrates and a liquid crystal layer (a liquid crystal composition) held between them and display letters, figures, drawings, pictures and the like owing to electro-optical effects. As to the driving mode of the liquid crystal displays, there are known TN (twisted nematic) method, STN (supertwisted nematic) method, FLC (ferroelectric liquid crystal) method, IPS (in-plane switching) method and the like. In the liquid crystal displays, color display is carried out by passing light transmitted by a liquid crystal layer at each pixel through a color filter. Leading displays as such liquid crystal displays have been changed to displays for desktop monitors from conventional displays for notebook-sized personal computers, and the liquid crystal displays are indispensably used in large-screen televisions. Therefore, it is increasingly desirable to improve the liquid crystal displays in performance characteristics and reduce their production cost.
The organic light emission displays are so-called self-luminous displays in which organic light-emitting layers are driven by application of an electric current to emit light. As the organic light emission displays, there are displays in which color display is carried out by utilizing the emission of lights having a plurality of colors, respectively, depending on materials for the organic light-emitting layers themselves, and displays in which color display is carried out by forming all pixels by the use of a whitely luminescent material and passing light emitted by the luminescent material through color filters provided for two or more colors, respectively.
In displays in which color display is carried out by the use of color filters, among the above-mentioned displays, color display is carried out by utilizing color filters of two or more colors (typically, the three primary colors, i.e., red (R), green (G) and blue (B), or complementary colors for the three primary colors, i.e., cyan (C), magenta (M) and yellow (Y)). Materials for such color filters, production processes and properties for display of the color filters, and the like are explained below.
Although the explanation is given herein by taking the case of the color filters of the liquid crystal display, it applies also to the color filters of other displays. The color filters used in the liquid crystal display are composed of a striped or dotted pattern having the above-mentioned colors, i.e., red, green and blue. The color development in each of these color filters is due to a component obtained by dissolving or dispersing a dye or pigment as a coloring agent in an acrylic base polymer, and the color filter is formed by applying the base polymer containing the coloring agent on a glass substrate to a thickness of about 1 micron. It will be increasingly desirable to improve the color filters in performance characteristics and reduce their production cost.
As the coloring agent of the color filter, a dye dissolved as molecules in the base polymer is superior from the viewpoint of contrast. However, a liquid crystal composition is contaminated in some cases with the dye owing to the diffusion of the dye itself into liquid crystals from the color filter, and the contamination causes nonuniform display. Moreover, a color change caused by the photo-deterioration of the dye is fatal to the color filter, and a pigment superior in light resistance is unavoidably used in current color filters at the cost of contrast. JP-A-2002-309135 discloses a technique for improving contrast by reducing the particle size of a pigment. The technique disclosed in this reference, however, involves a pigment atomization step and only increases the production cost by increasing the number of steps.
JP-A-8-6242 discloses a technique for improving contrast by using latent pigments as coloring agents. In the technique disclosed in JP-A-8-6242, pigments are solubilized by molecular modification in a step of dispersing them in a base polymer and each pigment modified is restored to its original state by removing a substituent used for the molecular modification, by light and heat treatments after forming the base polymer. However, in the case of the latent pigments, the dissolution of an acrylic resin as base polymer in a solvent is not sufficient. Therefore, JP-A-2002-72465 discloses a method for making up for a deficiency in coloring density by a combination of latent pigments and pigments.
As the base polymer, an acrylic resin is generally used because of a combination of a pigment dispersion method and photo-etching. Not only the acrylic resin but also other polymers used as the base polymer can absorb light in the vicinity of 400 nm near the ultraviolet region, in the visible region (wavelength: 400 nm to 800 nm) and are often colored yellow. This coloring becomes more remarkable owing to heat treatment such as post-baking. This phenomenon of coloring causes a decrease in transmittance for blue color in a color filter and hence is a problem to be solved for realizing the improvement of the color filter in performance characteristics.
There are base polymers which are transparent in the visible region, but such base polymers are often unusable from the viewpoint of compatibility with a solvent, for example, they are often unusable because their solubility in a solvent for them, which is absolutely necessary in a pigment dispersion method, is low. In addition, even if they are soluble in the solvent, this solubility does not result in the reduction of coloring owing to the influence of, for example, a sensitizer necessary to photo-setting base polymers. Such a phenomenon of coloring remarkably affects the color purity of the color filters with respect also to a combination of the color filters and a backlight. That is, it is expectable to improve the color purity by replacing a cold-cathode tube with a light emitting diode (LED) as a backlight, but this replacement cannot be expected to be very effective when there is the phenomenon of coloring.
As described above, production processes of color filters are broadly divided into a pigment dispersion method and a method using dyes, according to the kind of a coloring agent. In practice, the former, i.e., the pigment dispersion method is adopted for mass production in view of, for example, light resistance. In the pigment dispersion method, a photosensitive base polymer solution containing a red pigment dispersed therein is applied on a substrate having a black matrix formed thereon and is dried, after which etching is conducted by the use of a photomask to form red colored portions in the form of stripes. Subsequently, the same procedure as above is repeated except for using a blue pigment and then a green pigment, to produce color filters. Therefore, the number of steps for the production is large.
As described above, the production of color filters by the pigment dispersion method involves many steps. This is a cause for an increase of the production cost. When a coming increase in the size of a liquid crystal display is considered, an increase in the cost of a production apparatus itself is possible. JP-A-11-305032 discloses a process for producing color filters by adopting an ink jet method. This production process can be adopted only when dyes are used as coloring agents, and it is still disadvantageous, for example, in that a nozzle for ink jet is choked when pigments are used.
A first object of the present invention is to solve the problems in color filters described in the case of the above prior arts, i.e., the contrast decrease, the color purity decrease, the decrease in transmittance for blue color, and the like, and provide a colored composition for color filter which has made it possible to improve productivity in the production of color filters and achieve the improvement of the color filters in performance characteristics and the reduction of their production cost at the same time. A second object of the present invention is to provide a display having a high contrast, a high color purity and an increased transmittance for blue color.